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LANs, VLANs, and Trunking Overview

Jun 12, 2025

Overview

This lecture explains local area networks (LANs), the concept of broadcast domains, and how virtual local area networks (VLANs) and trunking help manage and separate different network segments on switches.

Local Area Networks (LANs) and Broadcast Domains

  • A LAN is a group of devices within the same broadcast domain, usually connected through a switch.
  • Devices on different, unconnected switches are in separate broadcast domains and cannot communicate directly.
  • Managing separate switches for different broadcast domains increases complexity and costs.

Introducing VLANs

  • VLANs (Virtual Local Area Networks) allow segmentation of a switch into multiple broadcast domains.
  • Each port on a switch can be assigned to a specific VLAN (identified by a number), isolating traffic between VLANs.
  • Devices on the same VLAN can communicate, even if assigned to different physical ports on the same switch.

VLANs Across Multiple Switches and Trunking

  • VLAN trunking (802.1Q standard) enables communication of the same VLAN across multiple switches using a single physical connection.
  • A VLAN tag is added to Ethernet frames to identify their VLAN as they traverse the trunk link.
  • 802.1Q trunking allows up to 4,094 VLANs over a single trunk link.
  • The older ISL trunking method is now obsolete.

VLAN Tagging and Native VLAN

  • VLAN tags are inserted into Ethernet frames to indicate VLAN membership.
  • The native VLAN is a special VLAN on a trunk port whose traffic is not tagged.
  • Both switches on a trunk must have the same native VLAN configured to avoid errors.

Layer 2 vs. Layer 3 Switching

  • Layer 2 switches forward traffic based on MAC addresses; Layer 3 switches can also route traffic using IP addresses.
  • Layer 3 switches combine switching and basic routing in a single device, often using switched virtual interfaces (SVIs) for inter-VLAN routing.

VoIP, Data, and VLAN Applications

  • Voice over IP (VoIP) and data can share the same physical network using separate VLANs for traffic separation.
  • Trunking allows both voice and data VLANs to travel over the same cable, assigning phones and computers to different VLANs for optimal performance.

Key Terms & Definitions

  • LAN (Local Area Network) — A network of devices in the same broadcast domain.
  • Broadcast Domain — A network segment where all devices can reach each other with broadcast traffic.
  • VLAN (Virtual LAN) — Logical segmentation of a switch that creates separate broadcast domains on the same hardware.
  • Trunk (802.1Q trunk) — A single physical link carrying multiple VLANs between switches, using VLAN tags.
  • VLAN Tag — Identifier in an Ethernet frame marking VLAN membership; 12 bits allow 4,094 VLANs.
  • Native VLAN — VLAN whose traffic is not tagged over a trunk link.
  • Layer 2 Switch — Switch forwarding based on MAC addresses.
  • Layer 3 Switch — Switch with added ability to route between VLANs using IP addresses.
  • SVI (Switched Virtual Interface) — Virtual interface for routing traffic between VLANs on a Layer 3 switch.

Action Items / Next Steps

  • Review switch configuration for VLAN, trunk, and native VLAN settings.
  • Practice configuring SVIs on a Layer 3 switch for inter-VLAN routing.
  • Read documentation on 802.1Q trunking and VLAN setup.

Full transcript